Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Phosphorus and phosphorus compounds have several characteristics that provide flame retardant properties: a high radical quenching ability, which may limit the chemistry that propagates a flame; and char combustion products that may form a char layer, which physically covers the underlying material and protects it from burning.
Adding more phosphorus to a polymer improves flame retardancy, because it increases the eventual thickness of the phosphate char layer that results when the phosphorus is combusted. A concentrated form of phosphorus is red phosphorus, but it may be challenging to form into a granular powder for mixing with polymers, and it may release toxic phosphine gas upon contact with moisture. For example, phosphorus has been employed in a two component mixture with silica gel and found to have flame retardant activity. However, such mixtures may be challenging to form free-flowing granular powders suitable for handling, and may still be susceptible to contact with moisture.
It would be desirable to make thin polymer coatings using red phosphorus as a char former, since such films would be very efficient at preventing the underlying polymer from combusting. However, phosphorus particle average diameters are typically >100 micrometers, which may limit polymer film thickness, since such particles will stick out of films thinner than the particle size. Such particles may also be exposed at the surface of thicker polymer articles. Such exposed particles are undesirable since they may react with water to form toxic phosphine gas, in addition to other potentially negative functional and cosmetic effects on the polymer article. The two component phosphorus mixture with silica gel may be unsuited to polymer thin films and polymer surfaces because the phosphorus may still include typical particle average diameters >100 micrometers.
The present disclosure appreciates that incorporating phosphorus as a flame retardant into polymers, particularly thin polymer films, may be a challenging endeavor.